The present disclosure relates to a developing device configured to perform developing of an electrostatic latent image using toner and to an image forming apparatus including the developing device.
There is an image forming apparatus such as a multifunction peripheral, a copier, a printer, a facsimile, or the like, which develops an electrostatic latent image with toner so as to perform printing. Further, there is an image forming apparatus that uses a developer containing magnetic carrier and toner (so-called two-component developer). It is not appropriate to permit a magnetic brush to directly contact with a photoreceptor drum in view of image quality and the like. Therefore, the applicant has provided an image forming apparatus including a developing device using a method in which a developing roller is disposed to be opposed to the photoreceptor drum so as to carry toner, a magnetic roller opposed to the developing roller forms a magnetic brush, only the toner is moved to the developing roller by the magnetic brush so as to form a thin layer of the toner on the developing roller, and hence an electrostatic latent image is developed without permitting the magnetic brush to contact with the photoreceptor drum (this method is also referred to as “touchdown development” or “hybrid development”). This method is more advantageous in various points such as image quality, print speed, toner life, prevention of carrier scattering, than a one-component development method or the conventional two-component development method.
In the touchdown development method described above, when performing the development of an electrostatic latent image, an AC voltage (for example, approximately 1 to 2 kV as peak-peak voltage) is applied to the developing roller so that charged toner is caused to fly. As the peak-peak voltage of the AC voltage applied to the developing roller is larger, the toner can fly more sufficiently so that density of the developed toner image is apt to be higher and that the electrostatic latent image can be developed more efficiently.
However, when the peak-peak voltage of the AC voltage applied to the developing roller is becomes too high, discharge occurs between the developing roller and the photoreceptor drum. When discharge occurs, a surface potential of the photoreceptor drum is disturbed, and hence image quality may be deteriorated. In addition, when the discharge is large, a micro hole may be formed on the photoreceptor drum so that the photoreceptor drum may be damaged.
Therefore, there is a case where a discharge start voltage (peak-peak voltage at which discharge starts) is detected as for the AC voltage applied to the developing roller. In other words, there is a case where a potential difference between the photoreceptor drum and the developing roller at which discharge starts is detected. In the detection of the discharge start voltage, it is detected that current flows in the developing roller or the photoreceptor drum so as to determine whether or not discharge has occurred.
Here, because the toner is charged, when the toner moves (flies) from the developing roller to the photoreceptor drum, the charge is moved. Because of this movement of toner, it may be falsely detected that discharge has occurred. Therefore, it is preferred not to supply toner to the developing roller during a discharge start voltage detection mode, so that the developing roller does not carry the toner.
On the other hand, the AC voltages to be applied to the developing roller and the magnetic roller (for supplying toner to the developing roller) may be generated by a transformer. Further, in order to simplify the circuit and to reduce manufacturing cost, the voltage to be applied to the developing roller and the voltage to be applied to the magnetic roller may be generated on a secondary side of the same transformer. In other words, a transformer having two or more secondary windings may be used.
Using this transformer, it is impossible to apply AC voltage only to the developing roller. When AC voltage is applied to the developing roller, AC voltage is also applied to the magnetic roller. Therefore, when AC voltage is applied to the developing roller in the discharge start voltage detection mode, AC voltage is also applied to the magnetic roller, and hence the toner moves from the magnetic roller to the developing roller.
Then, there is a problem that when using a transformer having two or more secondary windings, the toner is supplied to the developing roller in the discharge start voltage detection mode. In addition, there is a problem that there is high possibility of false detection of the discharge start voltage when the toner is supplied to the developing roller.